1. Field of the Invention
The present invention relates to an image forming apparatus and a sub-line control section for an image forming apparatus, which drive elements of emitting light or emitting heat in order to form images or to print data.
2. Description of Related Art
In general, an image forming apparatus such as a printer prints each line. And, each line includes plural dots. For example, an LED printer prints each line with an LED head, which comprises plural elements of LED's arrayed in a line. Meanwhile, in order to increase resolution of an image formed or printed, a method called smoothing is adopted. This is a method of printing a sub-line between each of main lines, so as to compensate for space between main lines. In order to smooth lines, that is, slanted lines in an image, it is necessary to form dots in sub-lines smaller than dots in main lines. Therefore, it is necessary to make energy for emitting light at sub-lines less than energy for emitting light at main lines.
Said elements for emitting light, emit light with energy which is generally proportional to product of magnitude and time, of a pulse current given to the elements for emitting light. And, in conventional art, the magnitude of a pulse current is fixed. Therefore, the time of a pulse current, that is, pulse width of a current is changed as a place to light is changed to a main line or to a sub-line, so as to change magnitudes of dots at a main line or at a sub-line (c.f. JP11-291550).
However, in the conventional printer mentioned above, there was a following problem. A pulse current needs a certain length of time to ascend or to descend. And, the time to ascend or to descend, influences the elements for emitting light. That is, the elements do not emit light until the pulse current ascends to a stable level. And, the elements become unstable while the pulse current descends. Therefore, when printing is performed in a high speed, dots in each line become unstably printed. And, each of dots are printed with different magnitudes. As a result, quality of printing decreases.